A flexible polymer substrate typically includes an organic polymer such as polyimide, and is often referred to as a tape substrate, or simply as a film substrate. For example, a tape substrate is typically fabricated by positioning an adhesive material layered between a polyimide or other flexible polymer and a metal layer (e.g., copper foil, or aluminum foil). As a first step, a surface of a polymer layer is covered with a film adhesive that is protected by a removable, protective plastic sheet. After punching through-holes at desired locations, the protective sheet covering the adhesive layer is peeled off, thereby exposing the adhesive surface. A thin copper foil is laminated to the adhesive surface, thereby creating a 3-layer polymer/adhesive/copper flexible tape substrate having multiple holes in the polymer layer and no holes in the metal layer, such a 3-layer polyimide/adhesive/copper foil flexible tape substrate provided by the 3M Company, St. Paul, Minn. 55144. When the polymer tape substrate provides sufficient adhesion to the metal foil, the adhesive layer can be excluded.
The metal foil is then patterned to form a metal pattern that includes metal pads over the through-holes, and solder resist over the metal pattern except in metal locations that will be bonded to, with the metal pattern side used for the top of the substrate for later having an IC affixed thereto. The through-holes are generally at least partially filled in a pre-solder ball metal filling process with a metal in an effort to fill the topside of the through-holes (directly under the metal pads) commonly referred to as the neck of the holes before adding solder balls into the partially filled holes from the bottomside of the substrate. Bonding wires connected to bond pads on the IC device are connected to the metal pad locations without solder resist thereon, which couple the bond pads to the metal in the partially filled holes. Solder ball are then added onto the metal into the holes to couple to the bond pads via the bond wires to form a ball grid array (BGA). Mold compound may then be added to encapsulate the IC, the bond wires, and all or part of the top surface of the polymer substrate.
Such BGA packages are low cost packages that can be used as a low cost CSP (Chip Size/Scale Package) with a small packaging area to permit high-density surface mounting of various types of IC devices. These BGA packages are commonly assembled onto a mother board using a solder reflow process, that generally involves a reflow temperature of 200° C. to around 260° C. The solder balls from the BGA package form solder joints following reflow with conductive contacts on the mother board.
Even with pre-solder ball metal partial filling of the through-holes, the necks represent a reliability problem area for the BGA package. One known problem with such BGA packages is lifting of the solder balls out from the holes, such as due to heat induced warpage resulting from coefficient of thermal expansion (CTE) differences during the reflow process. Another problem with such BGA tape packages is the relatively high cost for the solder ball process. What is needed is a low cost polymer substrate-based IC device that eliminates solder ball related problems.